Yahoo! has posted a message in the Yahoo! Publisher Network console, saying:

Network Update
You may have noticed a decrease in impressions in your Yahoo! Publisher Network account recently. This is due to a partial outage that occurred during routine maintenance. We are actively addressing the issue and expect improvements by next week. Thanks for your patience.

So by "next week" we can expect improvements, but what about this week?

YahooSarah has also come in to a WebmasterWorld thread saying:

I wanted to chime in briefly to let you know that we are aware of the problems you are experiencing and are working to resolve as soon as possible. Apologies for the inconvenience.